A reduction gear train is disposed within a power tool, such as the above-mentioned hammer drill, for reducing the rotational output speed of an electric motor that serves as a drive source. The reduction gear train is housed inside a gear housing. A lubricant such as grease is coated on the reduction gear train. Because lubricant-coated parts hold heat that is caused by operation of the power tool, the viscosity of the coated lubricant decreases or it liquefies; because the lubricant is pressurized by agitation, the lubricant tends to leak to the exterior through small gaps between joining surfaces of the gear housing.
Typically, an oil seal, a gasket, or the like is interposed between the joining surfaces of the gear housing to prevent leakage of the lubricant; in addition, a technique for preventing oil leakage without using a separate component such as an oil seal is disclosed in a patent document (Japanese Patent No. 5073449). In this patent document, a technique for preventing lubricant from going through small gaps owing to capillary action and from leaking to the exterior of the housing is disclosed.